Search: clarify file filter description (#682)

This PR clarifies the description of the `file:regexp-pattern` search
filter. The regexp is not anchored by default, and can match against any
substring of the path.

Resolves this thread:
https://sourcegraph.slack.com/archives/C05R619V4F8/p1727689383280839

Author: jtibshirani

docs/code-search/queries/index.mdx

@@ -51,8 +51,8 @@ The following filters can be used on all searches (using [RE2 syntax](https://go
 |   **repo:regexp-pattern** <br /> **repo:regexp-pattern@rev** <br /> **repo:regexp-pattern rev:rev**<br />_alias: r_   |   Include results from repos whose path matches a specified regexp-pattern. The repo path is `github.com/myteam/abc` or `code.example.com/xyz` depending on your repo host. If the regexp ends in `@rev`, search that revision instead of the default `main` branch. `repo:regexp-pattern@rev` is equivalent to `repo:regexp-pattern rev:rev`   |   [`repo:gorilla/mux testroute`](https://sourcegraph.com/search?q=repo:gorilla/mux+testroute) <br/> [`repo:^github\.com/sourcegraph/[email protected] mux`](https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24%40v3.14.0+mux&patternType=keyword)   |
 |**-repo:regexp-pattern** <br /> _alias: -r_   |   Exclude results from repositories whose path matches the regexp   |   `repo:alice/ -repo:old-repo`   |
 |   **rev:revision-pattern** <br /> _alias: revision_   |   Search a revision instead of the default branch. `rev:` can only be used in conjunction with `repo:` and may not be used more than once. See our [revision syntax](#repository-revisions) documentation to learn more   |   [`repo:sourcegraph/sourcegraph rev:v3.14.0 mux`](https://sourcegraph.com/search?q=repo:sourcegraph/sourcegraph+rev:v3.14.0+mux&patternType=keyword)   |
-|   **file:regexp-pattern** <br /> _alias: f_   |   Only include results in files whose full path matches the regexp   |   [`file:\.js$ httptest`](https://sourcegraph.com/search?q=file:%5C.js%24+httptest) <br /> [`file:internal/ httptest`](https://sourcegraph.com/search?q=file:internal/+httptest)   |
-|   **-file:regexp-pattern** <br /> _alias: -f_   |   Exclude results from files whose full path matches the regexp   |   [`file:\.js$ -file:test http`](https://sourcegraph.com/search?q=file:%5C.js%24+-file:test+http)   |
+|   **file:regexp-pattern** <br /> _alias: f_   |   Only include file results whose full path matches the regexp. The regexp is unanchored by default: to match against the entire path, use regexp anchors like `^README.md$`.   |   [`file:\.js$ httptest`](https://sourcegraph.com/search?q=file:%5C.js%24+httptest) <br /> [`file:internal/ httptest`](https://sourcegraph.com/search?q=file:internal/+httptest)   |
+|   **-file:regexp-pattern** <br /> _alias: -f_   |  Exclude file results whose full path matches the regexp. The regexp is unanchored by default: to match against the entire path, use regexp anchors like `^README.md$`.    |   [`file:\.js$ -file:test http`](https://sourcegraph.com/search?q=file:%5C.js%24+-file:test+http)   |
 |   **content:"pattern"**   |   Set the search pattern with a dedicated parameter. Useful when searching literally for a string that may conflict with the [search pattern syntax](#search-pattern-syntax). In between the quotes, the `\` character will need to be escaped (`\\` to evaluate for `\`)   |   [`repo:sourcegraph content:"repo:sourcegraph"`](https://sourcegraph.com/search?q=repo:sourcegraph+content:"repo:sourcegraph"&patternType=keyword)   |
 |   **-content:"pattern"**   |   Exclude results from files whose content matches the pattern. |   [`file:Dockerfile alpine -content:alpine:latest`](https://sourcegraph.com/search?q=file:Dockerfile+alpine+-content:alpine:latest&patternType=keyword)   |
 |   **select:_result-type_** <br /> **select:repo** <br /> **select:commit.diff.added** <br /> **select:commit.diff.removed** <br /> **select:file** <br /> **select:content** <br /> **select:symbol._symbol-type_** <br /> **select:file.owners** _(Experimental)_   |   Shows only query results for a given type. For example, `select:repo` displays only distinct repository paths from search results, and `select:commit.diff.added` shows only added code matching the search. See [language definition](/code-search/queries/language#select) for full list of possible values   |   [`fmt.Errorf select:repo`](https://sourcegraph.com/search?q=fmt.Errorf+select:repo&patternType=keyword)   |